The invention relates to screens for use in a screen printing system for printing onto electronic devices.
Screen printing can be used for creating patterns on the surface of a target electronic structure, such as wafers, printed wiring boards (PWBs), multichip modules, and hybrid substrates. Screen printing machines also have other applications, such as use in printing ink onto various articles, including clothing, paper, etc.
The screens have predetermined patterns and form openings through which a paste material can be forced for deposition on a target surface underneath the screen. Three types of screens have been used: mesh or emulsion screens, rigid metal mask stencils, and flexible metal mesh stencils. The mesh or emulsion screen is made of wire mesh patterned by polymer emulsion. Stainless steel, polyester, and nylon are common materials for the wire in the mesh screen. Typically, the mesh screen is attached to a screen frame (which can be aluminum) under tension with a bonding adhesive.
The stencil (either metal mask stencil or flexible metal mesh stencil) is made from a solid metal foil that is chemically or laser etched to a desired pattern. The stencil is then attached to a frame. The metal foils are usually made of metallic materials such as brass, stainless steel, copper, beryllium copper, and nickel. For flexible metal mesh stencils, the foil is not directly bonded to the frame. Rather, a flexible mesh structure bridges the foil and is attached to the frame by adhesives.
After print cycles, the screen is cleaned to remove excess material. Typically, screens are cleaned by generally scraping and wiping the gross amount of paste off the screen, followed by rinsing the screen with proper solvents, which can include a trichloroethane/alcohol blend solvent or a freon-type solvent. Screen manufacturers, as well as printing paste manufacturers, sometimes recommend or supply cleaning solvents specially formulated for the screen. Cleaning systems available with screen printers typically clean only one side of the screen--the side on which the paste is applied or the opposite side (back of the screen).
Screen printing machines also use "teaching" sheets to determine if the screen printing assembly (including the mounted screen, the squeegee, and a test target substrate) is properly aligned. The teaching sheet includes a transparent film that is affixed to a frame. The teaching sheet is mounted over a test target substrate and loaded into the screen printing machine to perform test prints. Patterns printed onto the transparent film can then be analyzed to determine whether the machine is properly aligned. After each use, the teaching sheet is typically cleaned and re-used.
Typical cleaning techniques can remove most but not all of the printed materials, especially the material stuck to walls of apertures in the screen. As a result, traces of the printing material may be left on the screens or teaching sheets.